Ivan Mercolli

Extremely thin oceanic crust in the Proto-Indian Ocean : Evidence from the Masirah Ophiolite, Sultanate of Oman

The Masirah Ophiolite is a good example of thin oceanic crust. Below pillow lavas and a sheeted dike complex with a relatively normal thickness of 1-1.5 km, the gabbroic lower crust barely exceeds 500 m in thickness. In spite of this reduced thickness, the oceanic crust preserves all members of a model ophiolite in a coherent lithostratigraphic sequence. The crust was formed during the uppermost Jurassic (circa 150 Ma) when the Indian-Madagascar plate separated from the African-Arabian plate and is therefore related to the opening of the coeval Somali basin. Geological relationships indicate that this portion of oceanic crust was formed at a ridge-transform intersect. The peculiarly reduced thickness of the gabbro layer is interpreted as the result of a weak magma supply at the edge of a ridge segment, rather than the consequence of a tectonic thinning. The cooling effect due to the vicinity of two large continental lithospheric blocks (Indian-Madagascar and African-Arabian plates) during this initial stage of the oceanization might have been an additional factor contributing to the reduction of the crustal thickness.

OFF-RIDGE ALKALINE MAGMATISM AND SEAMOUNT VOLCANOES IN THE MASIRAH ISLAND OPHIOLITE, OMAN

Abstract The Masirah ophiolite offers an unique opportunity to study well preserved small seamount structures. Obducted seamounts have not been described up to now, and from the present-day ocean floor they are almost exclusively known from bathymetric studies. The thin oceanic crust of the Masirah ophiolite was formed at a ridge-transform intersect in Upper Jurassic time. It was overprinted and reworked by a major intra-oceanic tectono-magmatic event at mid-Cretaceous time, that has been well dated owing to the presence of interstratified sedimentary rocks (late Hauterivian to early Barremian, c. 130-125 Ma). This mid-Cretaceous magmatism produced alkaline volcanic rocks ranging in chemistry from alkalibasalts to rhyolites. Volcanism occurred in a NW-SE extensional regime. Small, elongate submarine volcano structures (seamounts) developed within widespread alkalibasaltic pillow lava and pillow breccia deposits, which are interfingered with deep-marine pelagic sediments. The volcanoes reached a maximum of a few kilometres in diameter and a few hundred metres in height. The seamounts are built up of basic to acid subvolcanic stock- or sheet-like intrusions, several generations of dikes, vent agglomerates and pyro- to epiclastic deposits. The latter range from coarse breccias to finely stratified lapilli and record explosive volcanism in a deep marine environment. In the magma chambers under the volcanoes local differentiations to trachytic and rhyolitic members took place. The alkaline rocks show a pronounced ocean island basalt (OIB) character indicating the considerable contribution of a mantle plume source (hotspot). As cause of the volcanism we propose a combination of original transform setting followed by drift past the Marion hotspot during the major plate tectonic reorganization between Greater India, Madagascar and Africa starting in mid-Cretaceous time.

Early Cretaceous intra-oceanic rifting in the Proto-Indian Ocean recorded in the Masirah Ophiolite, Sultanate of Oman

Abstract The Masirah Ophiolite (Sultanate of Oman) was part of an oceanic basin (Proto-Indian Ocean) formed by the break-up of Gondwana in Late Jurassic times similar to the Somali basin. It was obducted onto the Arabian continental margin in the Early Paleocene, 100 Ma after its formation. Hence, it is possible to investigate the different tectonic and magmatic processes that have affected the oceanic lithosphere during these 100 Ma. Tithonian ridge magmatism, tectonism and hydrothermal alteration are responsible for the formation of the oceanic crust of the Masirah Ophiolite. In the Early Cretaceous (Hauterivian-Barremian), after 20 Ma of normal drift and subsidence, the oceanic lithosphere underwent extensional tectonics and renewed magmatism. Geometry, kinematics, intrusion mechanisms and related sedimentation during this intra-oceanic rifting are widely described and illustrated by field observations. Exhumation of deep-seated oceanic lithosphere, alkaline volcanism, intrusion of a hornblende gabbro-dolerite-granite suite and uplift of crustal blocks to sea level with the unconformable deposition of platform carbonates are the processes characterising this intra-oceanic rifting. The Hauterivian-Barremian age of oceanic rifting coincides with an important reorganisation of the motion of the Indian plate relative to Africa, Antarctica and Australia. We interpret the rifting recorded in the Masirah Ophiolite as the local response to the motion of the Indian plate due to the opening of the South Atlantic and the spreading in the Eastern Indian Ocean.

The Uplift History of the Precambrian Crystalline Basement of the Jabal J’alan (Sur Area)

Field relations and geochronological data from the Precambrian crystalline complex of the Jabal J’alan area allow the reconstruction of its uplift history. K/Ar cooling ages on biotites date the uplift of the whole complex (metamorphics and plutonics) through the 300°C isotherm at 820 Ma. Sedimentary rocks on top of the crystalline complex in the Northern Jabal J.alan area indicate uplift during Late Proterozoic and Jurassic times. Fission track data on apatite indicate the complex reached the 100°C isotherm during Upper Jurassic times (146 Ma). After Upper Cretaceous denudation of the exposed crystalline rocks, Maastrichtian and Tertiary sediments were deposited. Tertiary burial to depths of 2-3 km partially reset the apatite ages. Final uplift of the crystalline block took place in the Upper Oligocene.

Mechanical anisotropy control on strain localization in upper mantle shear zones

Mantle rocks at oceanic spreading centers reveal dramatic rheological changes from partially molten to solid-state ductile to brittle deformation with progressive cooling. Using the crustal-scale Wadi al Wasit mantle shear zone (SZ, Semail ophiolite, Oman), we monitor such changes based on quantitative field and microstructural investigations combined with petrological and geochemical analyses. The spatial distribution of magmatic dikes and high strain zones gives important information on the location of magmatic and tectonic activity. In the SZ, dikes derived from primitive melts (websterites) are distributed over the entire SZ but are more abundant in the center; dikes frommore evolved, plagioclase saturated melts (gabbronorites) are restricted to the SZ center. Accordingly, harzburgite deformation fabrics show a transition from protomylonite (1100°C), mylonite (900–800°C) to ultramylonite (<700°C) and a serpentine foliation (<500°C) from the SZ rim to the center. The spatial correlation between solid-state deformation fabrics and magmatic features indicates progressive strain localization in the SZ on the cooling path. Three stages can be discriminated: (i) Cycles of melt injection (dunite channels and websterite dikes) and solid-state deformation (protomylonites-mylonites; 1100–900°C), (ii) dominant solid-state deformation in harzburgite mylonites (900–800°C) with some last melt injections (gabbronorites) and ultramylonites (<700°C), and (iii) infiltration of seawater inducing a serpentine foliation (<500°C) followed by cataclasis during obduction. The change of these processes in space and time indicates that early dike-related ridge-parallel deformation controls the onset of the entire strain localization history promoting nucleation sites for different strain weakening processes as a consequence of changing physicochemical conditions.